Monolithically integrated sensing device and method of manufacture

ABSTRACT

A method of manufacturing a monolithic compound semiconductor sensing device includes epitaxially depositing a signal conditioning epitaxy on a substrate surface, providing a well in the signal conditioning circuit and exposing the substrate surface, and epitaxially depositing a sensor within the well.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a divisional of U.S. patentapplication Ser. No. 09/664,187 filed Sep. 18, 2000, which claims thebenefit of U.S. Provisional Application No. 60/219,551 filed Jul. 20,2000.

FIELD OF THE INVENTION

[0002] The present invention relates to sensing devices. Moreparticularly, the present invention relates to monolithicallyintegrated, compound semiconductor sensing devices and methods ofmanufacturing such devices.

BACKGROUND OF THE INVENTION

[0003] Sensing devices are used in various applications, some of whichsubject these sensing devices to harsh operating conditions such astemperature extremes, dirty environments and electromagnetically activesurroundings. For example, in the automobile industry sensing devicesare used in crankshaft position detection, wheel speed sensors,brushless electric motors, and other environments subject to widetemperature variations. Automotive sensor specifications require sensorsthat continually operate with significant change in performance acrosstemperatures ranging from −40° C. to over 200° C.

[0004] One conventional sensing device is a magnetic field sensingdevice. A magnetic field sensing device can be fabricated using asilicon semiconductor Hall sensor that is typically integrated into asilicon semiconductor signal conditioning circuit. A signal conditioningcircuit is used to convert the output signal from the Hall sensor into asignal that is useful for its intended application.

[0005] Silicon Hall sensors possess the disadvantages of lowersensitivity and signal-to-noise ratio because of the low electronmobility of silicon. Accordingly, these silicon devices lack precisionand require demanding signal conditioning circuitry. Additionally,traditional silicon circuitry is not suitable for use over a widetemperature range, particularly at temperatures greater than about 150°C. At higher temperatures, issues such as leakage current and parasiticconductance significantly impact the operation of silicon circuitry.

[0006] Sensing devices utilizing compound semiconductor technology arealso available. Manufacture of these devices, however, involves separatefabrication of the integrated circuit components. In a standardmanufacturing process, a magnetic field sensor, such as a Hall sensor, amagnetoresistor or a magnetotransistor, is fabricated by depositing atleast one layer, usually multiple layers, of compound semiconductormaterial such as InSb on a substrate material. Typically, each of thelayers used to form the magnetic field sensor has a differentcomposition or contains a different dopant or dopant level than theadjacent layers. The materials used to form the layers in themanufacture of magnetic field sensors may be deposited as thin epitaxialfilms utilizing a process such as metal organic chemical vapordeposition (MOCVD), vapor phase epitaxy (VPE), liquid phase epitaxy(LPE) or molecular beam epitaxy (MBE). In a separate process, anapplication specific integrated circuit (ASIC) to provide signalconditioning is manufactured.

[0007] A prior art method for manufacturing a sensing device is shown inFIGS. 1A-1D. Referring to FIG. 1A, a substrate 10 having a top surface12 may comprise a compound semiconductor material such assemi-insulating GaAs or other suitable III-V compound semiconductormaterial. Referring now to FIG. 1B, a sensor element 14 is shown on thesurface 12 of the substrate 10. The sensor element may be anyconventional sensor element, including a magnetic sensor, such as a Halleffect or magnetoresistive type sensor. Such a structure can be formedby one or more epitaxial layers. As shown in FIG. 1C, a separatesubstrate 20 is provided for manufacture of a signal conditioningcircuit, which may be fabricated using either silicon or compoundsemiconductor technology.

[0008]FIG. 1D shows an example of signal conditioning circuit layersincorporating a heterojunction bipolar transistor (HBT) structure 22deposited on substrate 20. As is known in the art, an HBT structure mayinclude active layers such as an emitter layer 24, a base layer 26 and acollector layer 28. These layers may be etched or otherwise treated inconventional ways to form the elements of the signal conditioningcircuit. Additional layers, such as dielectric layers and metallizationlayers (not shown), may be formed to provide contacts forinterconnection with another device, and to interconnect elements of thesignal conditioning circuit with one another. After separate fabricationof the magnetic sensor element and the signal conditioning circuit, thetwo structures are interconnected by wirebonding the structures togetherto form a completed sensing device.

[0009] Separate fabrication of the sensor and the signal conditioningcircuit has several drawbacks. Wirebonding uses valuable space on boththe sensor and signal conditioning circuit. Wirebonded devices tend toexhibit lower reliability than integrated devices that do not requirewirebonding of the separate elements. In addition, the wire-bondingprocess is labor-intensive, costly and increases the size of thefinished assembly.

[0010] It would be useful to provide a sensing device capable ofoperating over a wider temperature range than devices fabricated usingsilicon semiconductor signal conditioning circuits. It would also beadvantageous to provide a process for manufacturing a compoundsemiconductor sensing device having a sensor and signal conditioningcircuit that is less labor intensive, more reliable and less expensivethan methods that involve separate device fabrication andinterconnection.

SUMMARY OF INVENTION

[0011] Accordingly, one aspect of the present invention provides amonolithically integrated, compound semiconductor sensing deviceincluding a signal conditioning circuit and a sensor element. In oneembodiment, a sensor element including one or more compoundsemiconductors is epitaxially deposited over a first portion of thesubstrate surface. In another aspect of the invention, the semiconductorlayers which will be used to form the signal conditioning circuit (the“signal conditioning epitaxy”) is formed by epitaxial deposition of oneor more compound semiconductors on second portion of the substratesurface.

[0012] In a preferred embodiment, the signal conditioning epitaxyinclude one or more III-V compound semiconductor. Similarly, the sensorelement includes at least one III-V compound semiconductor, whichtypically is different from the compound semiconductor used in thesignal conditioning epitaxy. Preferably, the surface of the substrate isformed from GaAs.

[0013] In a preferred embodiment, the sensor is a magnetic sensor, suchas a Hall effect or magnetoresistive sensor. In one aspect of theinvention, the sensor is in direct contact with the substrate surface.In another aspect of the invention, a buffer layer may be disposedbetween the substrate surface and the sensor. Similarly, a buffer layermay be disposed between the signal conditioning epitaxy and thesubstrate surface.

[0014] Another aspect of the invention involves a method ofmanufacturing a monolithic compound semiconductor sensing device. Themethod includes forming signal conditioning epitaxy adapted to form asignal conditioning circuit on a substrate surface and providing a wellin the signal conditioning epitaxy to expose the substrate surface. Inanother aspect of the invention, a sensor is formed in the wellstructure.

[0015] In one embodiment of the invention, formation of the wellstructure within the signal conditioning circuit involves etching thewell within the signal conditioning epitaxy after deposition of thesignal conditioning epitaxy. According to a further embodiment of theinvention, formation of the well within the signal conditioning circuitinvolves masking the substrate surface, depositing the signalconditioning epitaxy over the mask and removing the mask to provide awell.

[0016] In still another embodiment of a the invention, a method ofmanufacturing a monolithic compound semiconductor sensing device isprovided which includes forming a sensor on a substrate surface andproviding a well in the sensor to expose the substrate surface. A signalconditioning epitaxy is formed within the well formed in the sensor.

[0017] According to one embodiment of the invention, the step ofproviding a well in the sensor includes etching the well in the sensor.In a further embodiment of the invention, the step of providing a wellin the sensor epitaxy includes masking the substrate surface, depositingthe sensor over the mask and removing the mask to provide a well.

[0018] Preferably, the method of the present invention involves formingthe signal conditioning epitaxy first, and then later epitaxiallydepositing the sensor in a well formed in the signal conditioningcircuit. In general, the materials used to form sensor epitaxy, such asInSb, are not stable at the high temperatures required to form thesignal conditioning epitaxy. Therefore, it is preferred that the signalconditioning epitaxy is formed first, and the sensor element is formedafter deposition of the signal conditioning epitaxy.

[0019] The methods according to this aspect of the invention preferablyfurther include the step of forming all or part of a signal conditioningcircuit from the signal conditioning epitaxy. Most preferably, this stepincludes forming connections on the monolithic structure between thesensor element and the signal conditioning circuit.

[0020] Compared to devices using silicon sensors, the integrated devicesof the present invention provide higher sensitivity, improvedsignal-to-noise ratio, greater robustness against electrostaticdischarge, and substantially no switching noise. The present inventionalso has the advantage of greater flexibility in providing features tothe magnetic field sensing device. This signal conditioning circuit caninclude essentially any elements which can be used in such a circuit,such as, for example, amplifiers, power supply components such asvoltage regulators and overvoltage protection components such as zenerdiodes. Monolithically integrating the sensor and the signalconditioning circuit in one die provides several other advantages.

[0021] The monolithically integrated manufacturing method of the presentinvention provides a simpler and less expensive method of manufacturingsensing devices which utilize a sensor and a signal conditioning circuitthan conventional processes involving separate fabrication of thecomponents. For example, the pick-and-place portion of the deviceassembly operation is simplified since only one die incorporating bothcomponents requires handling. The wire-bonding of the signalconditioning and sensor components is less expensive, due to the factthat only the input and output connections of the components need to bewired. In the conventional process, in which the components areseparately fabricated, die-to-die interconnections also need to beseparately wire-bonded.

[0022] It will be understood that fewer manufacturing steps provides anopportunity for improved product yields and reduced manufacturingerrors. In addition, device reliability is improved due to the fact thatthe finished device has fewer interconnections. Additional features andadvantages of the invention will be set forth in the following detaileddescription. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIGS. 1A-1D show a cross-sectional view of prior art method ofmaking a sensing device;

[0024]FIG. 2 shows a cross-sectional view of an embodiment of a deviceaccording to the present invention;

[0025]FIG. 3 shows a cross-sectional view of an alternative embodimentof a device according to the present invention;

[0026] FIGS. 4A-4I show a cross-sectional view of an alternative methodof making a device in accordance with an embodiment of the presentinvention; and

[0027] FIGS. 5A-5D show a cross-sectional view of alternative method ofmaking a device in accordance with the present invention.

DETAILED DESCRIPTION

[0028] Reference will now be made in detail to the present preferredembodiment of the invention, an example of which is illustrated in theaccompanying drawings. FIG. 2 shows a substrate 100, which is preferablya semi-insulating compound semiconductor material such as GaAs.Substrate 100 has a substrate surface 102. In a preferred embodiment,signal conditioning epitaxy 104 adapted to form a signal conditioningcircuit are epitaxially deposited on the substrate surface. Preferably,the signal conditioning epitaxy includes bipolar transistor structure,such as a heterojunction bipolar transistor (HBT), which is well-knownin the art. For example, an HBT structure includes at least a collectorlayer 106, a base layer 108, and an emitter layer 110, as is known inthe art. It will be appreciated that the HBT structure can includeadditional layers such as a subcollector and other layers, depending onthe requirements of the particular device.

[0029] According to one aspect of the invention, a well structure 112can be formed in the signal conditioning epitaxy 104, and preferably thewell structure exposes the substrate surface 102. Sensor epitaxy 114 isformed in the well structure 112. Preferably, the sensor epitaxy 114 isadapted to form a magnetic sensor such as a Hall sensor or amagnetoresistive sensor, which are known in the art. For example, thesensor epitaxy 114 may include at least one InSb layer. However, theinvention is not limited to any particular material, and other sensormaterials such as GaAS, InAs and HgCdTe could be utilized to form thesensor.

[0030] In another embodiment shown in FIG. 3, a substrate 200 having asurface 202 is provided. The substrate 200 is preferably asemi-insulating material such as GaAs or other suitable III-V compoundsemiconductor material. A sensor epitaxy 204 is formed on the substratesurface. Preferably, the sensor epitaxy 204 is adapted to form amagnetic sensor such as a Hall effect sensor or magnetoresistive sensor.In a preferred embodiment, the sensor is formed from a material such asInSb or InAs. The sensor epitaxy 204 includes a well structure 206formed therein, and the well structure 206 preferably exposes thesurface 202 of the substrate 200. A signal conditioning epitaxy 208 isdeposited within the well structure 206. Preferably the signalconditioning epitaxy includes a bipolar transistor structure includingat least a collector layer 214, a base layer 212, and an emitter layer210.

[0031] Referring now to FIGS. 4A-4I, a further method of manufacturing acompound semiconductor sensing device is shown. As shown in FIG. 4A, asubstrate 300 having a surface 302 is provided. The substrate 300 ispreferably a semi-insulating compound semiconductor material such asGaAs. Referring to FIG. 4B, a signal conditioning epitaxy 304 isdeposited on the substrate surface 302. As shown in FIG. 4B, preferablythe signal conditioning epitaxy includes a bipolar transistor structurehaving a collector layer 306, a base layer 308 and an emitter layer 309.

[0032] Referring now to FIG. 4C, a well structure 310 is provided in thesignal conditioning epitaxy 304. The well structure 310 can be formed bya variety of methods known in the art. For example, the well structure310 can be formed by first depositing the signal conditioning epitaxy304 and then etching the well structure 310 in the signal conditioningepitaxy 304 by methods known in the art. Alternatively, the wellstructure 310 can be provided in the signal conditioning epitaxy 304 bymasking the substrate surface 302 according to methods known in the art.After masking the substrate surface 302, the signal conditioning epitaxy304 can be deposited. The mask is then removed to provide the wellstructure 310 in the signal conditioning epitaxy 304. Preferably, thewell structure 310 exposes the substrate surface 302.

[0033] Referring now to FIG. 4D, sensor epitaxy 312 is provided in thewell structure 310. Preferably the sensor epitaxy 312 includes at leastone epitaxial layer of InSb or InAs adapted to form a magnetic sensorsuch as a Hall effect sensor or magnetoresistive sensor. As shown inFIG. 4D, the sensor epitaxy may be deposited in the well structure 310and over the signal conditioning epitaxial structure 304. Those portionsof the sensor epitaxy overlying the signal conditioning structure 304are then removed as, for example, by etching. Alternatively, the signalconditioning epitaxial structure 304 can be selectively masked prior todeposition of the sensor epitaxy according to methods known in the art,and the sensor epitaxy 312 can be deposited only in the well-structure310.

[0034] After formation of the signal conditioning structure, the wellstructure 310 and the sensor epitaxy 312 within the well structure 310,further processing steps known in the art may be performed to fabricatethe sensing device and to form the signal conditioning circuit from thesignal conditioning epitaxy. For example, photolithographic techniquesknown in the art can be used to form a sensor mesa 316 as shown in FIG.4E. Referring to FIG. 4F, further photolithographic processing can beused to etch mesas 318, 320 and 322 in the signal conditioning epitaxy304.

[0035] As shown in FIGS. 4G and 4H, metal contacts and interconnects 324and dielectric layers 326 can be formed by techniques known in the art.The interconnects 324 include one or more interconnects extendingbetween the sensor elements and elements and one or more elements of thesignal conditioning circuit, and may also include one or moreinterconnects extending between elements of the signal conditioningcircuit itself. As shown in FIG. 4I, additional metal contacts 328 canbe deposited over the dielectric layer. In use of the device thecontacts provide connections between the signal conditioning circuit andexternal devices. Further processing steps known in the art of formingintegrated circuits can be performed to fabricate a device in accordancewith the present invention.

[0036] In another embodiment of the invention shown in FIGS. 5A-5D, amethod of making a sensing device is shown. FIG. 5A shows a substrate400 having a surface 402. Preferably, the substrate 400 is asemi-insulating material such as GaAs or another suitable III-V compoundmaterial. FIG. 5B shows a sensor epitaxy 404 deposited on the substratesurface. As in the previous embodiments, preferably the sensor epitaxyforms a magnetic sensor, such as a Hall effect or magnetoresistivesensor. A well 406 is provided in the sensor epitaxy, and preferably thewell 406 exposes the substrate surface 402, as shown in FIG. 5C.

[0037] As in the previously described embodiment, the well 406 can beprovided by methods known in the art. For example, the well 406 can beprovided by etching a well in the sensor epitaxy 404 after deposition ofthe sensor epitaxy 404. Alternatively, the well can be provided byselectively masking the substrate surface 402 prior to deposition of thesignal processing epitaxy 404 and removing the masking to provide thewell 406 in the sensor epitaxy.

[0038] Referring to FIG. 5D, a signal conditioning epitaxy 408 isdeposited in the well 406. The signal conditioning epitaxy 408 can bedeposited in the well 406 and over the sensor epitaxy 404, and thesignal conditioning epitaxy can be selectively etched to provide asignal conditioning circuit mesa 410 within the well 406, as shown inFIG. 5D. Alternatively, the sensor epitaxy 404 can be selectively maskedto leave the well open prior to deposition of the sensor epitaxy so thatthe signal conditioning epitaxy 408 is deposited only in the well. Suchmasking would provide a signal conditioning circuit mesa 410 in thewell.

[0039] As in the previous embodiment described in FIGS. 4A-4I,additional processing steps known in the art of integrated circuitprocessing may be performed to provide a finished sensing device. Forexample, photolithographic steps can be performed to form mesas in thesensor element, and metal contacts, and dielectric layers can beprovided to interconnect the sensor and the signal conditioning circuit.A buffer layer can be provided between the substrate surface and thesensor and/or between the substrate and the signal conditioning circuit.

[0040] It will be appreciated that it may be preferable to first depositthe signal conditioning circuit prior to deposition of the sensorelement. This is so because certain materials for forming magneticsensors, such as InSb, are not stable at the higher temperaturesrequired to process the materials used to form the signal conditioningcircuit. However, InAs or other materials can be used in formation ofthe sensor element, which would possibly accommodate higher temperatureprocessing required to deposit an signal conditioning circuit.

[0041] One example of the epitaxial layers making up a bipolartransistor structure is provided in Table I. Of course, the layers couldbe formed using heterojunctions of different III-V materials, such asAlGaAs and InGaP. TABLE 1 Dopant Concentration Layer Material ThicknessRange (cm⁻³) Emitter n-GaAs  0.2-0.5 micron   1 × 10¹⁷ to   5 × 10¹⁸Base p-GaAS 0.04-0.15 micron >1 × 10¹⁹ Collector n-GaAs  0.3-2.0 micron<1 × 10¹⁸

[0042] The sensor epitaxy is preferably formed from at least one layerof InSb or InAs having a thickness of about 0.4 microns to about 1.6microns. Preferably, the sensor material has a mobility of about 35,000cm²/V-s or greater.

[0043] It will be apparent to those skilled in the art that variousmodifications and variations can be made to the device and method of thepresent invention without departing from the spirit or scope of theinvention. For example, the materials, dopants, dopant concentrations,and thickness of the layers making up the various bipolar transistor,buffer and sensor elements can be varied to achieve the desired deviceproperties. Also, it not essential to deposit the various layers of thesignal conditioning epitaxy as uniform layers. For example, mesastructures can be formed by selectively depositing one or more of thelayers in the signal conditioning epitaxy over only a part of anotherlayer in the signal conditioning epitaxy. Similarly, the processingconditions for fabricating the devices of the present invention, such astemperature, can be varied. Thus, it is intended that the presentinvention cover the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents.

1. A method of manufacturing a monolithic compound semiconductor sensingdevice comprising: epitaxially depositing a signal conditioning epitaxyon a substrate surface; providing a well in the signal conditioningcircuit and exposing the substrate surface; and epitaxially depositing asensor within the well.
 2. The method of claim 1, wherein the step ofproviding well in the signal conditioning circuit includes etching thewell.
 3. The method of claim 1, wherein the step of providing a well inthe signal conditioning circuit includes masking the substrate surface,epitaxially depositing the signal conditioning circuit over the mask andremoving the mask to provide a well.
 4. The method of claim 2, whereinthe signal conditioning epitaxy includes a heterojunction bipolartransistor structure.
 5. The method of claim 4, wherein the sensor is amagnetic sensor.
 6. The method of claim 5, wherein the magnetic sensoris a magnetoresistive or Hall sensor.
 7. The method of claim 3, whereinthe signal conditioning epitaxy includes a heterojunction bipolartransistor structure.
 8. The method of claim 7, wherein the sensor is amagnetic sensor.
 9. The method of claim 8, wherein the magnetic sensoris a magnetoresistive or Hall sensor.
 10. A method as claimed in claim 1further comprising the step of forming a signal conditioning circuitfrom said signal conditioning epitaxy so that said signal conditioningcircuit is electrically connected to said sensor.
 11. A method ofmanufacturing a monolithic compound semiconductor sensing devicecomprising: epitaxially depositing a sensor on a substrate surface;providing a well in the sensor and exposing the substrate surface; andforming a signal conditioning epitaxy within the well.
 12. The method ofclaim 11, wherein the step of providing a well in the sensor includesetching the well.
 13. The method of claim 12, wherein the signalconditioning epitaxy includes a heterojunction bipolar transistorstructure.
 14. The method of claim 13, wherein the sensor is a magneticsensor.
 15. The method of claim 14, wherein the magnetic sensor is amagnetoresistive or Hall sensor.
 16. The method of claim 11, wherein thestep of providing a well in the sensor includes masking the substratesurface, depositing the sensor over the mask and removing the mask toprovide a well.
 17. The method of claim 16, wherein said signalconditioning epitaxy includes a heterojunction bipolar transistorstructure.
 18. The method of claim 17, wherein the sensor is a magneticsensor.
 19. The method of claim 18, wherein the magnetic sensor is amagnetoresistive or Hall sensor.
 20. The method of claim 11, furthercomprising the step of forming a signal conditioning circuit from saidsignal conditioning epitaxy so that said signal conditioning circuit iselectrically connected to said sensor.